Apparatus and method for input clock signal detection in an asynchronous transfer mode interface unit

ABSTRACT

In a data processing system have a master-state data processing unit and at least one slave-state data processing unit, the data processing units can be provided with an asynchronous transfer mode interface unit for transferring data cells there between. The interface unit provides and receives signals formatted in the Utopia protocol. The interface unit clock signals from an external source or from the master processing unit. To insure the integrity of the data transfer through the interface unit, a clock signal detection system is provided. When the applied clock signal is not detected, a reset signal and an interrupt signal are generated.

RELATED APPLICATIONS

[0001] This application claims priority under 35 USC 119(e)(1) of Provisional Application Serial No. 60/237,237, filed Oct. 02, 2000.

[0002] APPARATUS AND METHOD FOR AN INTERFACE UNIT FOR DATA TRANSFER BETWEEN PROCESSING UNITS IN THE ASYNCHRONOUS TRANSFER MODE; U.S. patent application Ser. No. (Attorney Docket No. TI-31779); filed on even data herewith; invented by Shakuntala Anjanaiah and Natarajan Seshan; and assigned to the assignee of the present application: APPARATUS AND METHOD FOR AN INTERFACE UNIT FOR DATA TRANSFER BETWEEN A HOST PROCESSING AND A MULTI-TARGET DIGITAL SIGNAL PROCESSING IN AN ASYNCHRONOUS TRANSFER MODE; U.S. patent application Ser. No. (Attorney Docket No. TI-33430; filed on even date herewith; invented by Martin Li, Jay Reimer, Shakuntala Anjanaiah, Natarajan Seshan and Patrick Smith; and assigned to the assignee of the present application: and; APPARATUS AND METHOD FOR AN INTERFACE UNIT FOR DATA TRANSFER BETWEEN DATA PROCESSING UNITS IN THE ASYNCHRONOUS TRANSFER MODE AND IN THE I/O MODE; U.S. patent application Ser. No. (Attorney Docket No. TI-33534); filed on even date herewith; invented by Shakuntala Anjanaiah, Roger Kyle Castille, and Natarajan Seshan; and assigned to the assignee of the present application are related applications.

BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION

[0003] This invention relates generally to data processing systems and, more particularly, to data processing systems having a host processor and at least one digital signal processor. An interface unit is inserted between the host processor and the digital signal processor(s) to facilitate the exchange of data there between.

2. BACKGROUND OF THE INVENTION

[0004] As the requirements for computational power have increased, one data processing system that has been increasing employed to meet these requirement includes a host processing system that controls one or more digital signal processors. The host processor is typically a microprocessor, but can be a digital signal processor. The host processor has the flexibility to respond to a wide variety of conditions and provide an appropriate response. The digital signal processors provide specialized capabilities that permit complex but repetitive tasks to be performed very rapidly. Thus, one or more digital signal processing units, operating under control of a master processing unit, can respond to a wide variety of computational intensive requirements. However, the host processor and the digital signal processor(s) may not be directly compatible and may even be fabricated by different manufacturers. In order to permit the interchange of data between incompatible components or components which can exchange data with difficulty, standard signal protocols have been agreed upon to provide the requisite commonality. As an example, the asynchronous transfer mode defines signals that facilitate the exchange of data signal groups between a host processor and a digital signal processor. A protocol has been provided for the Universal Test and Operations Phy Interface (UTOPIA) for the asynchronous transfer mode (ATM) (UTOPIA) Level 2 Interface to conform to the ATM Forum standard specification af-phy-0039.000 as well as other applicable standards. The UTOPIA protocol defines the interface between the Physical Layer (PHY) and the upper layer module such as the ATM Layer and various management entities. This definition allows a common PHY in ATM systems across a wide range of speeds and media types. The ATM cell or packet that is transferred in this protocol includes 53 bytes with a 5 byte header and a 48 byte payload in an 8-bit transfer mode, or 54 bytes with a 6 byte header and a 48 byte payload in a 16-bit transfer mode.

[0005] The UTOPIA protocol defines the exchange of data signals between master processing unit and the slave processing unit. When the interface unit is operating in the slave mode, externally applied clock signals synchronize the transfer of data through the interface unit. In the absence of the external clock signals, the integrity of the data cells transferred through the interface unit can not assured.

[0006] A need has therefore been felt for apparatus and an associated method having the feature that the transfer of data cells through a interface unit using the UTOPIA protocol is assured. . It is another feature of the apparatus and associated method the interface unit include a clock signal detection circuit to verify the presence of an external clock signal during a cell transfer. It is a still further particular feature of the apparatus and associated method that a reset signal and an interrupt signal be generated when the external clock signal is no longer present.

SUMMARY OF THE INVENTION

[0007] The aforementioned and other features can be accomplished, according to the present invention, by providing a digital signal processor configuration with an interface unit responsive to UTOPIA-defined signals. In the slave mode, the interface unit is responsive to external clock signals to insure the integrity of the data transferred to and from the interface unit. A programmable detection unit is provided in the interface unit to detect when the external clock signals are absent. In the absence of external clock signals, the interface unit is reset and an interrupt signal is transmitted to the central processing unit.

[0008] Other features and advantages of present invention will be more clearly understood upon reading of the following description and the accompanying drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a block diagram of the general data processing system capable of advantageously using the present invention.

[0010]FIG. 2 is a block diagram illustrating the signals generated by and signals received by the asynchronous transfer mode interface unit in the slave-transmit state according to the present invention.

[0011]FIG. 3 is a timing diagram for the signals received by and generated by the asynchronous transfer mode interface unit in the slave-transmit state shown in FIG. 2 according to the present invention.

[0012]FIG. 4 is a block diagram illustrating the signals generated by and received by the asynchronous transfer mode interface unit in the slave-receive state according to the present invention.

[0013]FIG. 5 a timing diagram for the signals received by and generated by the asynchronous transfer mode interface unit in the slave-receive state mode shown in FIG. 4 according to the present invention.

[0014]FIG. 6 is a block diagram illustrating the signals generated by and received by the asynchronous transfer mode interface unit in the master-transmit state according to the present invention.

[0015]FIG. 7 is a timing diagram for the signals received by and generated by the asynchronous transfer mode interface unit in the master-transmit state mode shown in FIG. 6 according to the present invention.

[0016]FIG. 8 is a block diagram illustrating the signals generated by and received by the asynchronous transfer mode interface unit in the master-receive state according to the present invention.

[0017]FIG. 9 is a timing diagram for the signals received by and generated by the asynchronous transfer mode interface unit in the slave-receive state mode shown in FIG. 8 according to the present invention.

[0018]FIG. 10A illustrates the asynchronous transfer mode Utopia protocol signals with a master-state data processing unit in a transmit mode and a plurality of slave-state data processing units in a receive mode, while FIG. 10B illustrates the asynchronous transfer mode Utopia protocol signals with a master-state data processing unit is a receive mode and a plurality of slave-state data processing units in a transmit mode.

[0019]FIG. 11 is a block diagram of an implementation of the Utopia interface unit according to the present invention

[0020]FIG. 12 illustrates the contents of the interface control register according to the present invention

[0021]FIG. 13 is a flow chart illustrating the operation of an EVENT signal in the Utopia interface slave transmit mode according to present invention.

[0022]FIG. 14 is a flow chart illustrating the operation of an EVENT signal in the Utopia interface slave receive mode.

[0023]FIG. 15 is a block diagram for the determination of the missing UTOPIA clock signal and the generation of a reset signal according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] 1. Detailed Description of the Figures

[0025] Referring to FIG. 1, a block diagram of the data processing system 1 capable of advantageously incorporating the present invention is shown. The data processing system includes at least one digital signal processing unit 100 through 10N, a communication bus 110, and a master processing unit 120. Each digital signal processing unit 100 through 10N includes a central processing unit (or digital signal processing unit core) 10, memory unit 12, a direct memory access unit 14, and a UTOPIA interface unit 18. The interface unit 18 of each digital signal processing unit 100 through 10N exchanges signals with the bus 100. Master processing unit 120 also exchanges signals with the communication bus 110. The interface unit 18 exchanges signals with the direct memory access unit 14. The direct memory access unit 14 exchanges signals with the memory unit 12 and, subsequently, with the core processing unit 10. Note that the master processing unit 120 can be digital processing unit such digital signal processing unit 100.

[0026] Referring to Table 1, the required signals for the ATM Forum Technical Committee's UTOPIA Level 2, Version 1.0 (af-phy-0039.000) are listed. Note that optional signals, not included in Table 1, are identified. TABLE 1 ATM Controller ATM Controller Master Slave Signal Name (Dir) (Dir) Transmission Mode UXCLK In In UXADDR {4:0} Out In UXCLAV In Out UXENB Out In UXSOC Out Out UXDATA {15:0} Out Out Receive Mode URCLK In In URADDR {4:0} Out In URCLAV In Out URENB Out In URSOC In In URDATA {15:0} In In

[0027] In the transmit slave mode, the Utopia signals have the following meaning. The UXCLK signal is a clock input signal driven by the master processing unit. The UXDATA signals and the transmit control signals are synchronized with this UXCLK signal. The UXADDR {4:0} is 5-bit address signal group generated by the master processing unit. This address signal group is used to select one of a plurality (up to 31) slave processing units in the system. The UXCLAV signal is a transmit cell available status output signal of the slave processing unit. For a cell level handshake, a 0 logic level indicates that the slave interface unit does not have a complete data cell for transmission, while a logic 1 indicates that the slave interface unit has a complete data cell to transmit. The UXENB signal is a transmit interface enable signal input signal. This signal is asserted low by the master processing unit to indicate that the slave processing unit should apply the first byte of valid data and the UXSOC (start-of-cell) signal in the next clock cycle. The UXSOC signal is the start of cell signal (active high) that is generated by the slave processing unit on the rising edge of the UXCLK signal to indicate that the first valid byte of the cell is available on the transmit data bus. The UXDATA {15:0} signals are provided by the slave processing unit during the transmission of on the transmit data bus on the UXCLK rising edge.

[0028] In the receive slave mode, the URCLK signal is a clock signal applied to the interface unit by the master processing unit. The receive data and control signals are sampled and are synchronous to this clock signal. The URADDR {4:0} signals are applied to the interface unit by the master processor and identify one of the slave units (up to 31) in the system. The URCLAV signal is the receive cell available output signal from the slave interface unit to indicate that the slave interface unit has space available to receive a cell from the master processing unit. In the handshake procedure, the 0 logic bit indicates that no space is available to receive a data cell from the master processing unit. The 1 logic bit indicates that space is available to receive a data cell from the master processing unit. The URENB signal is an active low signal generated by the master processing unit to enable the receive interface of the slave processor. This signal indicates that the slave interface unit is to sample URDATA signal and the URSOC signal during the next clock cycle or thereafter. The URSOC signal is generated by the master processing unit and indicates that the first valid byte of the data cell is available on the receive data bus for the slave processor to sample. The URDATA {15:0} signals are applied by the master processor to the data receive bus and sampled on the rising edge of the CLK signal.

[0029] As can be seen from Table 1, in the master mode, the UXCLAV/URCLAV and the UXENB/URENB signals are reversed in direction when compared to the counterpart slave signals. The reversal in direction is the result of the different role played by a master mode interface unit and a slave mode interface unit. Similarly, the UXADDR and URADDR signals have reversed directions between the master mode and the slave mode resulting from the fact that the polling takes place from the master mode. The interpretation of the signals remains the same.

[0030] Referring to FIG. 2, the signals applied to and generated by the UTOPIA interface unit 18 in the slave transmit mode are shown. The interface unit 18 includes two components, a processor 184 acting as a state machine, and a buffer memory unit 182. The processor 184 receives the UXCLK signal, the UXADDR {4:0} signal and the UXENB signal. The processor 184 generates the UXCLAV signal, the UXSOC signal and the UXDATA {15:0} signal. The processor 184 applies the WRD_RDY signal to the buffer memory unit and the processor 184 receives the DATA {31:0} signals and the CLAV signal from the buffer memory unit 184. The buffer memory unit 182 receives the WD_WR signal, the ADDR {31:0} signals, the data {31:0} signals and the ADDR {31:0} signals from the direct memory access unit 14. The buffer memory unit 182 applies the EVENT signal to the direct memory access unit 14.

[0031] Referring to FIG. 3, a timing diagram illustrating the relationship of the signals for the asynchronous transfer mode interface unit 14 in the transmit-mode depicted in FIG. 2 are shown. The signals are synchronized by the UXCLK signal. When a slave mode asynchronous transfer mode interface unit 18 detects its address on the UXADDR {4:0} lines, the processor will provide a UXCLAV signal to indicate whether or not a cell is available for transmission. After completion of the current activity, the master processor generates the address signal group, UXADDR {4:0}, and the UXENB signal. The slave processor then transmits the data over the conductors carrying the DATA {15:0} signals and by asserting the UXSOC signal.

[0032] Referring to FIG. 4, the exchange of signals between the interface unit 18 and the master processing unit and between the interface unit 18 and the direct memory access unit 14 is shown. The processor 184 of the interface unit 18 receives the URCLK signal, the URADDR {4:0} signals, the URENB signal, the URSOC signal, and the URDATA {15:0} signals from the master processing unit. The processor 184 applies the URCLAV to the master processing unit. The processor 184 applies the DATA {31:0} signals and the WD_WR to the buffer memory unit 182 and processor 184 receives the CLAV signal from the buffer memory unit 182. The buffer memory unit 182 applies the DATA {31:0} signals and the EVENT signal to the direct memory access unit 14 and the buffer memory unit 182 receives the ADDR {31:0} signals and the WD_RD signal from the direct memory access unit 14. In the slave receive mode, the DATA signals are transferred by the processor 184 to the buffer memory unit 182, and then to the direct memory access unit 14. The CLAV signal and the WD_WR signal permit the DATA signals to be transferred through the processor 184 to the buffer memory unit 182. The WD_RD signal permits the DATA signals to be transferred from the buffer memory unit 182 to the direct memory access unit 14

[0033] Referring to FIG. 5, a timing diagram illustrating the relationship of the signals for the asynchronous transfer mode interface unit 14 in the receive-mode depicted in FIG. 4 are shown. The signals are synchronized by the URCLK signal. The master processor applies the ADDR {4:0} signal group to the slave processors. The identified slave processor responds to the ADDR {4:0} signal with the appropriate CLAV signal. When an active CLAV signal is applied, the ADDR {4:0} signals are reapplied along with the ENB signal. The slave starts receiving data along with an SOC signal. The DATA {15:0} signals continue to be received until the cell has been completely transferred.

[0034] Referring to FIG. 6, the signals exchanged by the interface unit 18 in the master-transmit mode is shown. The processor 184 of the interface unit 18 receives the UXCLK signal and the UXCLAV signal. The interface unit 184 applies the UXADDR [4:0] signals, the UXENB signal, the UXSOC signal, and the UXDATA {15:0} signals to the slave processing unit. The processor 184 applies the WD_RD signal to the buffer memory unit 182 and processor 184 receives the DATA {31:0} signals and the CLAV signals from the buffer memory unit 182. The buffer memory unit 182 applies the EVENT signal to the direct memory access unit 14 and the buffer memory unit 182 receives the DATA {31:0} signals, the ADDR {31:0} signals, and the WD_WR signal from the direct memory access unit 14. In the master-state transmit state the DATA signals are transmitted from the direct memory access unit 14 to the buffer out memory unit 182, and then through the processor 184 to the external component. The WD_WR signal permits the DATA signals to be transmitted from the direct memory access unit 14 to the buffer memory unit 182. The CLAV signal and the WR_RD signal permit the DATA signals to be transferred from the buffer memory unit 182 to the processor 184 and, subsequently to the external component.

[0035] Referring to FIG. 7, the timing diagram for the Utopia interface unit in the master-transmit state of FIG. 6 is shown. The master-transmit state processor polls, with the UXADDR {4:0} signals, the slave devices in a round robin or in a fixed priority sequence. The processor 184 receives a UXCLAV signal, where appropriate, from a slave processor following the address, UXADDR {4:0} of the processor. The master-transmit processor then reapplies the address of the slave processor generating the UXCLAV signal along with the UXENB signal. During the next clock cycle, the processor begins transmission of the UXDATA {15:0} signals and the SOC signals to the slave processor. The transfer is continued until the entire cell has been transferred.

[0036] Referring to FIG. 8, the signals exchanged by the UTOPIA interface unit 18 in the master-receive mode are shown. The processor 184 of the interface unit 18 applies the URADDR {4:0} signals and the URENB signal to the slave processing unit and the processor 184 receives the URCLK signal, the URCLAV signal, the URSOC signal and the URDATA {15:0} signals from the slave processing unit. The processor 184 applies the DATA {31:0} signals and the WD_WR signals to the buffer memory unit 182 and the processor 184 receives the CLAV signal from the buffer memory unit 182. The buffer memory unit 182 applies the DATA {31:0} signals and the EVENT signal to the direct memory access unit 14 and the buffer memory unit 182 receives ADDR {31:0} signals and the WD_RD signal from the direct memory access unit 14.

[0037] Referring to FIG. 9, a timing diagram is shown for the signals of the asynchronous transfer mode interface unit in the master-receive state as illustrated in FIG. 8. The master processor 184 of polls the slave processors by applying the ADDR {4:0} signals to the address line. When a slave asynchronous transfer mode interface unit is available to receive the data signals, a CLAV signal is asserted during the next clock cycle. During the following clock cycle, the ADDR {4:0} signals of the slave unit generating the CLAV signal is reapplied to the bus along with the ENB signal. When a current interaction with the master processor is complete, the addressed slave processor transmits the DATA {31:0} signals and the SOC signal. The DATA {31:0} are transmitted until the entire cell has been transferred.

[0038] Referring to FIG. 10A and FIG. 10B, a data processing system is shown having a master-state data processing unit 91 and a plurality of slave-state data processing units 92A through 92N. In FIG. 10A, the master-state data processing unit 91 is in a transmit mode, while the slave-state data processing units 92A through 92N are in a receive mode. In FIG. 10B, the master-state data processing unit 91 is in a receive mode while the slave-state data processing units 92A through 92N are in a transmit mode. In FIG. 10A, the master data processing unit 91 (in the transmit mode) generates the UXCLK, the UXADDR, the UXENB, the UXSOC, and the UXDATA signals that which become the URCLK, the URADDR, the URENB, the URSOC, and the URDATA signals, respectively, when applied to the slave data processing units 92A-92N (in the receive mode). The URCLAV signals from the slave data processing units 92A-92N are applied to the master data processing unit 91 as the UXCLAV signal. In FIG. 10B, the master data processing unit 91 (in the receive mode) generates the URCLK, the URADDR, and the URENB signals that are applied to the slave data processing units 92A-92N (in the transmit mode as the UXCLK, the UXADDR, and UXENB signal respectively. The slave data processing units 92A-92N generate the UXCLAV, the UXSOC, and the UXDATA signals that are applied to the master data processing unit 91 as the URCLAV, the URSOC, and the URDATA signals, respectively.

[0039] Referring to FIG. 11, the implementation of the UTOPIA interface unit between the communication bus 110 and the direct memory access unit 14, according to the present invention, is shown. Data from the communication bus 110 is transferred through the interface input unit 181 to the interface input buffer memory unit 182. From the interface input buffer memory unit 182, the data signals are transferred through the direct memory access unit 14 to the memory unit(s) of the digital signal processing unit chip 100. The data from the memory units is transferred through the direct memory access unit 14 to the interface output buffer memory unit 183. The data is transferred from the interface output buffer memory unit 183 through the interface output unit 184 to the communication bus 110. The system logic 186 receives the INTERNAL CLOCK signal (as distinguished from the UTOPIA CLK signal), reshapes and deskews waveform and distributes the CLOCK signal to the rest of the UTOPIA interface unit 18. The configuration interface unit 185 receives the initialization signals and, by transmission of control signals to the other units of the UTOPIA interface unit 18 determines the mode in which the UTOPIA interface unit 18 operates. These control signals are stored in the interface control register 1851.

[0040] Referring to FIG. 12, the contents of the interface control register according to the preferred embodiment is shown. In the UREN/UXEN fields, a logic “0” indicates that the receive/transmit port is disabled, while a logic “1” indicates that the interface receive port is enabled. This designation is true in both the master and the slave modes. In the URMSTR/UXMSTR fields, a logic “0” indicates that the interface unit is operating in a slave (default) mode, while a logic “1” indicates that the interface unit is operating in a master mode. In the RUDC/XUDC fields, a user defined (i.e., standard or extended) data cell is specified for both the receive and the transmit operational modes. This field is used in the slave mode. In the SLID/SLEND field, this field identifies the address of the coupled processor unit in the slave mode. In the master mode, this field identifies the last of the processors coupled to the interface unit. In the UPM field, field identifies whether a polling takes place in a round-robin manner or from a fixed address. The U16M field determines whether data transfers are 8 bits or 16 bits for both the input and the output interfaces. The MPHY field determines whether the interface unit is coupled to a single processor (logic “0”) or to multiple processors. The ULB field determines whether the interface unit is in a loop-back mode. In the loop-back mode (i.e., logic “1”), the receive and transmit sections are coupled and the master is determined by the URMSTR/UXMSTR fields. The BEND field determines the data transfer in a big endian or little endian format.

[0041] Referring to FIG. 13, the operation of the (transmit) EVENT signal is illustrated. After initialization of the transmit portion of the UTOPIA interface unit in step 1300 or as part of the continuing operation of the interface unit, a determination is made in step 1301 whether a space for the storage of a complete data cell is available in the transmit buffer memory unit. When the determination is yes, then is step 1302 a transmit EVENT signal is applied to the direct memory access unit. In response to the generation of the EVENT signal, a data cell is transmitted through the direct memory access unit to the transmit buffer memory storage unit in step 1303. In step 1304, as soon as the transfer of the data cell has begun and the first word of the cell is written, the EVENT signal is cleared. Note that the EVENT signal is reasserted as soon as the first word is written and the buffer memory unit has space available. The immediate reassertion of the EVENT signal improves the interface unit throughput. The process than returned to step 1301 to determine whether space in the transmit buffer memory unit is available for storage of an entire data cell. When the determination is step 1301 is negative, the process returns to step 1301 and continues to cycle until space is available for the storage of an entire data cell.

[0042] Referring to FIG. 14, the operation of the event signal in the UTOPIA slave interface unit in the receive mode is illustrated. After initialization of the receive portion of the UTOPIA interface unit in step 1400 or as part of the operation of that portion of the interface unit, a determination is made whether a complete data cell is available in the receive buffer memory unit in step 1401. When the determination is yes, a receive EVENT signal is generated in step 1402. In response to the EVENT signal, the data cell in the receive buffer memory unit is transferred through the direct memory access unit instep 1403. In step 1404, as soon as the data cell transfer is begun with the reading of the first word, the EVENT signal is cleared. The process is then returned to step 1401 and is cycled until a complete data cell is stored in the receive buffer memory unit.

[0043] Referring to FIG. 15, the clock detection unit 1861 for determining when, in a slave mode of operation, the UTOPIA clock signal (UXCLK or URCLK) is missing and the UTOPIA interface unit should be reset. A clock detection circuit is provided, in the preferred embodiment of the present invention, for both the transmit channel and the receive channel of the Utopia interface unit. In clock detection unit 1861, the internal clock signal is applied to the input terminal of counter 1862. When the counter 1862 reaches a predetermined count, a RESET signal is applied to the UTOPIA interface unit. The predetermined count is determined by the value in the clock detection register 1863. The counter 1862 has the UTOPIA UXCLK or URCLK signal applied to the reset terminal of the counter unit 1862. The counter 1862 is configured such that a UXCLK/URCLK signal will cause the counter 1862 to be reset prior to the count reaching the predetermined count at which the RESET signal is generated. Therefore, the presence of the UXCLK or URCLK signal prevents the resetting the interface unit. In the preferred embodiment, a RESET signal results in the UTOPIA slave interface unit being reset to the values shown in TABLE 2. (The pin designations reference the signals that are applied to the pins.) TABLE 2 UTOPIA UTOPIA PINS SLAVE INTERFACE (DIR) RESET VALUES UXCLK In Low UXADDR In High UXCLAV Out Hi-Z UXENB In High UXSOC In Hi-Z UXDATA Out Hi-Z URCLK In Low URADDR In High URCLAV Out Hi-Z URENB In High URSOC In Low URDATA In Low

[0044] In addition, the when the absence of a UTOPIA clock signal is detected, an interrupt signal is sent to the central processing unit in order to provide a system-wide response.

[0045] 2. Operation of the Preferred Embodiment

[0046] The asynchronous transfer mode interface unit of the present invention is the interface unit between the direct memory access unit and the data processing system. In general, all of the data processing units of a data processing system can include an asynchronous interface unit using the UTOPIA protocol that is coupled to a bus coupling the data processing units. In some implementations, the data processing unit itself can be implemented to provide the asynchronous transfer mode signals thereby obviating the need for the interface unit in that data processing system. The actual transfer of the data signals between data processing systems in the asynchronous transfer mode is under the control of the same clock or synchronized clock signals. The data cells or packets that have been transferred or that are to be transferred are stored in the buffer memory unit. The memory unit provides a buffer between the clock frequency of the communication bus and the much higher frequency of the direct memory access unit.

[0047] The foregoing description has described the interface unit as including a buffer memory unit. In the preferred embodiment, the buffer memory unit is implemented by a first-in/first out memory unit. The memory unit is provided with the capacity to store two data cells. The communication bus causes the signals to be exchanged between the master unit and the slave unit to have a relatively slow clock speed. Because of the relatively slow clock speed of the communication bus, the filling or emptying of the buffer memory in the direction of the communication will be much slower than the filling and the emptying of the buffer memory unit in the direction of the direct access memory unit. Similarly, although the direct memory access unit can handle only one data transfer at a time, because of the difference in clock speed between the communication bus and the processing unit of which the direct memory access unit is a part

[0048] The EVENT signal is particularly useful in the efficient transfer of data cells. Because the operation of the data processing system of which the UTOPIA interface is a part is much faster than the rate at which data can be transferred over the communication bus, the transfer of data cells out of the transmit buffer memory unit and into the receive buffer memory unit can be essentially continuous.

[0049] While one important application of the present invention is the transfer of data signals between a host or master-state data processing unit (that includes a microprocessor) and at least one slave-state data processing unit (that typically includes a digital signal processor), this configuration can be reversed. In addition, the UTOPIA transfer mode interface unit can be added to each or a series of digital signal processing units coupled by a bus. One of the digital signal processing unit is selected as being the master-state machine and this processing unit controls the operation of all the digital signal processors.

[0050] The Utopia interface unit identifies the transfer of an incomplete ATM cell, sometimes referred to as a runt cell, when the SOC signal is set during a ATM cell transfer. In the software resolution, the runt cell can be resolved by transferring the runt cell to a higher level software procedure. In the preferred embodiment, the runt cell is overwritten by new data under hardware control.

[0051] Because the accurate transmission of data cells in the asynchronous transfer mode requires the application of a clock signal to the interface unit, the detection of absence of the clock signal is important for the operation of the system. The present invention provides a method whereby the internal or on-board clock is used as the time base to determine when the externally applied clock signals are applied in a timely manner. When the clock signal is not present, then the clock detection unit resets the interface unit and alerts the central processing unit by means of an interrupt signal. The interrupt signal permits the central processing unit to initiate recovery procedures.

[0052] While the invention has been described with respect to the embodiments set forth above, the invention is not necessarily limited to these embodiments. Accordingly, other embodiments, variations, and improvements not described herein are not necessarily excluded from the scope of the invention, the scope of the invention being defined by the following claims. 

What is claimed is:
 1. In a data processing system transferring data cells between processing units using an asynchronous transfer mode Utopia protocol, an interface unit comprising: apparatus for transferring data cells between the processing units, the transfer of data cells being controlled by an external clock signal; and a clock detection unit for detecting the absence of the external clock signal, the clock detection unit generating a interface reset signal when the external clock signal is not detected.
 2. The interface unit as recited in claim 1 wherein the clock detection unit includes a component for generating the interface reset signal when a external clock signal is not detected in a preselected time period.
 3. The interface unit as recited in claim 2 wherein the component is a counter, the counter counting the internal clock signals, the preselected time period determined by the number of internal clock signals.
 4. The interface unit as recited in claim 3 wherein the external clock signals are applied to a reset terminal of the counter.
 5. The interface unit as recited in claim 4 wherein the interface unit is operating in a slave mode.
 6. The interface unit as recited in claim 1, wherein the apparatus includes: an input interface unit; and an output interface unit; and wherein the buffer memory unit includes: an input buffer memory unit, wherein the transfer between the input buffer memory unit and the direct memory access unit is determined by a receive event signal; and an output buffer memory unit, wherein the transfer between the direct memory access unit and the output buffer memory unit is determined by a transmit event signal.
 7. The interface unit as recited in claim 6 wherein the receive event signal is generated when the buffer memory unit has a complete data cell stored therein, the receive event signal being cleared when transfer between the buffer memory unit and the direct memory access unit is begun, and wherein the transmit event signal is generated when the buffer memory unit has space for a complete data cell, the transmit event signal being cleared when the transfer of the data cell to the buffer memory unit from the direct memory access unit is begun.
 8. A method of insuring the accurate transmission of data through an interface unit acting in an asynchronous transfer mode, the interface unit having an external clock signal applied thereto, the method comprising: determining when the external clock signal is present; and when the external clock signal is not present, generating at least one control signal.
 9. The method as recited in claim 8 wherein generating a control signal includes generating at least one of a reset signal and an interrupt signal.
 10. The method as recited in claim 9 further comprising: counting the number of signals of an internal clock; resetting the count to zero by the external clock signals; and when the count reaches a predetermined number, generating the at least one control signal.
 11. The method as recited in claim 10 further comprising storing the predetermined number in a register.
 12. An asynchronous transfer mode Utopia interface unit for providing a interface between an external data processing unit and a direct memory access unit, the interface unit comprising: an input buffer memory unit, the input buffer memory unit providing data cells to the direct memory interface unit; an interface input unit, the interface input unit controlling the transmission of data cells from the external processing system to the input buffer memory unit; an output buffer memory unit, the output buffer memory unit receiving data cells from the direct memory access unit; an interface output unit, the interface output unit controlling transmission of data cells from the output buffer memory unit to the external processing system; and a clock detection unit, the clock detection unit generating a least one control signal when the external clock signal is not present.
 13. The interface unit of claim 12 wherein the clock detection unit includes a counter and a register, the counter having internal clock signals applied to an input terminal thereof, the counter having the external clock signal applied to a reset terminal thereof, the register coupled to the counter to determined the number of internal clock signals to be counted before the external clock signals was determined to be missing.
 14. The interface unit as recited in claim 12 wherein the input and output buffer memory units are first-in/first-out memory units, the first in/first out memory uits capable of storing can store at least two data cells.
 15. The interface unit as recited in claim 12 wherein the receive event signal is generated when the input buffer memory unit has a complete data cell stored therein, the receive event signal being cleared when transfer between the input buffer memory unit and the direct memory access unit is begun, and wherein the transmit event signal is generated when the output buffer memory unit has space for a complete data cell, the transmit event signal being cleared when the transfer of the data cell to the output buffer memory unit from the direct memory access unit is begun. 